1. Field of the Invention
The present invention relates to a magnetic head for recording information on a magnetic recording medium and/or reproducing information from a magnetic recording medium, and more particularly to a magnetic head which is composed of a pair of magnetic core half parts abut-welded to each other.
2. Description of the Prior Art
With the improvement of magnetic recording techniques, there have emerged more chances of using a magnetic, recording medium with a high coercive force in several devices such as a video tape recorder (VTR), a rotary head type digital audio tape recorder (R-DAT), a floppy disc drive (FDD), and still video equipment (SV). Such a magnetic recording medium thus requires a magnetic head which is made of magnetic materials with a high saturation magnetization in order to enable recording onto such a high coercive force medium. To improve the recording density on a recording medium surface, tracks and magnetic gaps must be made narrower.
In recent years, in lieu of a conventional ferrite head, a compound type head, what is called, a metal-in-gap type magnetic head has emerged as a main stream device. The metal-in-gap type magnetic head is composed of magnetic metal such as Sendust (Fe-Al-Si alloy) or amorphous magnetic metal such as amorphous Fe deposited nearby a magnetic gap of a core half part mainly made of ferrite by means of a vacuum film formation technique such as sputtering, vacuum evaporation, or ion plating. In such a magnetic head, there have been proposed as a magnetic gap material SiO.sub.2, TiO.sub.2, ZrO.sub.2, Cr, Al.sub.2 O.sub.3, TiC and Ta.sub.2 O.sub.5 and so on. Of these materials, SiO.sub.2 is most widely used, because it has a hardness of proper value, and a different color from that of a magnetic metal.
Such a compound type magnetic core is manufactured in accordance with the steps shown in FIGS. 1A-1F.
At first, a substantially rectangular parallelopiped type ferrite base 1 as shown in FIG. 1A is made of a Ni-Zn ferrite or Mn-Zn ferrite.
Next, as shown in FIG. 1B, a track ditch 1a for defining a track width of the magnetic core is formed on the ferrite base 1 at a given pitch P matching the track width. A section removed from the track ditch 1a has a substantial V-shaped character, but the form may be a U-shaped character type or a trapezoidal shaped type.
In addition to the ferrite base 1, a ferrite base 2 is prepared which also has a track ditch 1a formed thereon. On the ferrite base 2 is formed an escape ditch 2a for winding a coil on a magnetic core as shown in FIG. 1C.
Next, as shown in FIG. 1D, several .mu.m to several tens .mu.m thickness of a magnetic alloy film 3 with a high saturation magnetization is formed on the tops of the ferrite bases 1 and 2 by means of a film forming technique. In addition, a magnetic alloy used as a material of the magnetic alloy film 3 may be properly selected among Fe or Co alloys. For example, it is preferable to employ an Fe-Al-Si alloy (Sendust).
Next, on the magnetic alloy films 3 of the ferrite bases 1 and 2 are each formed an SiO.sub.2 film 4 for forming a magnetic gap for leaking a magnetic flux and/or picking up an external magnetic flux. As shown in FIG. 1E, the ferrite bases 1 and 2 are abutted and welded using a welding glass 5.
Then, the welded combination of the ferrite bases 1 and 2 is cut along a cut-away line 6 so as to obtain a magnetic core 7 as shown in FIG. 1F.
The magnetic core 7 has a structure wherein the magnetic core half parts 11 and 12 with magnetic metal films 3 formed on the opposite surfaces to one another are welded by the welding glass 5 through a magnetic gap G made of an SiO.sub.2 film. At a final stage, a coil winding is provided on the magnetic core 7 and then the resulting magnetic core 7 is mounted onto a base (not shown) so as to complete a magnetic head.
In the foregoing conventional magnetic head, in general, there are the following three problems.
The first problem is that reaction of a Sendust or amorphous magnetic metal film with welding glass results in producing a reaction layer. The characteristics of the magnetic head are degraded due to the reaction layer.
Moreover, the reaction layer is not preferable from an authentic point of view, and weakens the strength of the welding between the glass and the metal, resulting in making the overall magnetic head fragile. It is, therefore, necessary to select a magnetic gap material which allows occurrence of the reaction layer to be suppressed.
The second problem is that the welding glass is often difficult to penetrate into a gap between the magnetic cores because wettability of the materials used, such as ferrite, Sendust, or amorphous magnetic metals, by the welding glass for retaining and fixing the magnetic gap is diverse. For holding the strength of a head chip, therefore, it is necessary to select a magnetic gap material having a high wettability by the welding glass.
The third problem is that abrasion resistance in a gap material must be considered when a very narrow gap is made, because the gap material located between the ferrite bases needs respective abrasion resistances matching those of a ferrite or the metals such as Sendust, and amorphous metal. It is, therefore, necessary to select a magnetic gap material which has abrasion resistance allowing respective materials to be uniformly abraded.
Further details concerning these problems will be explained below.
First, a description will be directed to the reaction between the welding glass and the magnetic alloy film. When the ferrite bases 1 and 2 are welded by a welding glass 5, the SiO.sub.2 film 4 reacts with the welding glass 5, resulting in eroding the film 4, and the magnetic alloy film 3 reacts with the welding glass 5, resulting in eroding the alloy film 3. FIG. 2 is a plan view showing the circumference of a magnetic gap for illustrating the reaction between the welding glass and the magnetic alloy film. The foregoing reaction brings about an eroded portion 8 on the magnetic alloy film 3. The eroded portion serves to change the proper track width T of a magnetic gap G to advance along the magnetic gap G so as to widen the gap width, resulting in damaging the characteristics of the magnetic head.
To solve this shortcoming, as shown in FIGS. 3 and 4, on the magnetic alloy film 3 of the magnetic core there is formed a protective film 9 consisting of a metal which is excellent in corrosion resistance, such as Ta.sub.2 O.sub.5 or Cr. On the protective film 9 is formed the SiO.sub.2 film 4 which serves as a gap material. In the example shown in FIG. 3, the protective film 9 is formed on the overall surface, containing the portion facing the magnetic gap G of the track width T, of the magnetic alloy film 3. In the example shown in FIG. 4, the protective film 9 is formed on the magnetic alloy film 3 except for the portion facing the magnetic gap G. This example illustrates that the gap width is made narrower. In the manufacturing process, at first, the protective film 9 is formed on an overall surface of the magnetic alloy film 3 and then the portion of the film 9 facing the magnetic gap G is removed by some means such as lapping.
In the structure shown in FIG. 3, however, the variation of the gap widths becomes large, because the gap width of the magnetic gap G is defined on both thickness of the protective film 9 and the SiO.sub.2 film. And, in case of defining a narrower gap width, the protective film 9 cannot be thick enough to protect the magnetic alloy film 3, resulting in causing the eroded portion.
In the structure shown in FIG. 4, on the other hand, if there is a shift of the track due to a mismatching in position of magnetic core half parts 11 and 12 corresponding to the ferrite bases 1 and 2, an eroded portion is produced in the vicinity of an end portion along the track width of the magnetic gap G.
In either one of the structures in FIGS. 3 and 4, a step of forming the protective film 9 should be included in the manufacturing process. It results in increasing the number of steps in the process and making the manufacturing cost higher. Further, these structures also have the problem that the reaction between the welding glass and the magnetic alloy film cannot be effectively prevented.
Of the foregoing materials used for a magnetic gap, Cr is effective for preventing occurence of a reaction layer, though, Cr looks like the metal magnetic film material in color when seen with an optical microscope. In the manufacturing process of a magnetic head, accordingly, it is difficult to inspect a gap length using an optical microscope, thereby increasing the manufacturing cost of a magnetic head.
Next, a description will be directed to wettability of a gap material by a welding glass. Each wettability of the conventional gap materials by the welding glass is listed in Table 1.
TABLE 1 ______________________________________ Gap Material SiO.sub.2 TiO.sub.2 ZrO.sub.2 Al.sub.2 O.sub.3 Cr TiC Ta.sub.2 O.sub.5 ______________________________________ Contact Angle 75.degree. 45.degree. 70.degree. 70.degree. 40.degree. 50.degree. 70.degree. .theta. ______________________________________
Table 1 lists wettability of each magnetic gap material by welding glass (PbO-SiO.sub.2 -Bi.sub.2 O.sub.3 -B.sub.2 O.sub.3 glass) with a contact angle .theta.. The smaller the contact angle .theta. becomes, the better the wettability becomes. The contact angle .theta. was measured by the method shown in FIG. 5. That is, the method comprises the steps of forming a Sendust film 14 of a magnetic metal film with a high saturation magnetization to have a film thickness of 5 .mu.m on a ferrite base 13, forming a film 17 of each magnetic gap material on the formed film 14 to have a film thickness of 800 .ANG., placing a welding glass 18 with a given weight on the gap material 17, heating the welding glass at 570.degree. C. for forty minutes, and measuring a contact angle .theta. of the welding glass 18 against a film 17 of the magnetic gap material with a contact angle meter. The contact angle of the welding glass to the ferrite is 32.degree., and the contact angle of the welding glass to the Sendust is 62.degree.. As is obvious from Table 1, the magnetic gap materials except Cr and TiO.sub.2 are not excellent in wettability by the welding glass.
Next, a description will be directed to abrasion resistance of a magnetic gap material.
At first, Cr, Al.sub.2 O.sub.3, TiC, and ZrO.sub.2 are high in hardness and too high in abrasion resistance. The use of these materials, therefore, allows a magnetic gap portion of a magnetic core to be bulged. The bulge causes spacing loss, resulting an inferior output.
On the other hand, TiO.sub.2 is too low in hardness to cause a concave portion on the magnetic gap portion. The magnetic metal film material of the magnetic core is deformed and enters into the concave portion. Hence, the gap is capped so as to make the optical gap shorter, resulting in an inferior output.
SiO.sub.2 and Ta.sub.2 O.sub.5 offer a middle hardness level between TiO.sub.2 and Cr and the other materials and are suitable as a magnetic gap material with respect to abrasion resistance. As stated above, however, SiO.sub.2 is easy to react to the welding glass, and SiO.sub.2 and Ta.sub.2 O.sub.5 are inferior in wettability by the welding glass.
As set forth above, there are no conventional magnetic gap materials which can solve the foregoing shortcomings.